The emissions estimates shown here are those that are directly
emitted, which represent only a portion of the total PM-2.5 emissions found
in the air. Secondary formation of
fine particulates resulting from emissions of nitrogen oxide, sulfur dioxide,
volatile organic compounds, and other substances, is also a significant
source of PM-2.5.

Numbers may not add to totals due to rounding.

The
methodologies used to estimate emissions constantly evolve and undergo major
changes. These improved methods are
often used to revise estimates for previous years and, therefore, some
estimates in this table may not match estimates produced in previous
reports.